The 16-year-old lured Miss Meade to secluded garages behind a block of flats before stabbing her in the heart.

She then accompanied Miss Meade's boyfriend, Patrick Dawkins, to the cinema and was captured on a bus's CCTV camera re-enacting the killing.

Yesterday an Old Bailey judge told Ross, now 17, she would have to serve a minimum of ten and a half years in prison.

The court heard that Ross became infatuated with 19-year-old Mr Dawkins while babysitting for Miss Meade's 15-year-old and seven-year-old daughters and had begun a sexual relationship with him.

But Miss Meade, a 39-year-old welfare officer at a centre for excluded children, had told Dawkins, a former pupil at the centre, she believed she was carrying his child and gave him an ultimatum to leave Ross.

Fearing she would lose her lover, Ross, from Croydon, south London, decided to kill her rival.

Shortly before the murder in May last year she told her aunt: "I am going to stab her. Do you know where the heart is?"

Judge Gerald Gordon told Ross, who pleaded guilty to the murder: "Nothing can detract from the chilling horror of your offence.

"Over a period of three weeks, you had become infatuated with the boyfriend of Miss Meade.

"You may have believed that, when that relationship became sexual nine days before the killing, you had succeeded in taking him away from her.

"Something occurred which caused you to believe you had not.

"You were just not capable, until it was far too late, of understanding the full horror of what you were doing.

"This was a killing out of jealousy and to keep your man, as you saw it."

On the day of the murder, the court heard Ross had armed herself with a kitchen knife and had been picked up by Miss Meade following a phone call.

The pair drove to dark and secluded garages in South Norwood, south London, where Mrs Meade was stabbed twice in the heart, and repeatedly in the arm and hands as she tried to defend herself.

Prosecutor Sallie Bennett-Jenkins QC told the court: "On any view this was a chilling act, and the killing had been carefully premeditated by the defendant who had spoken of her intention to kill Ms Meade and met the deceased on that day, armed herself with a knife and ensured the deceased was to go to a dark and quiet location where she would be killed."

After the murder, Ross travelled with Mr Dawkins by bus to watch the X-Men film at the cinema and was filmed on CCTV re-enacting the stabbing.

Miss Bennett-Jenkins said: "It's clear she is demonstrating what she had in fact done to Tracy Meade, raising her hands and placing her hand to the area of the chest where Ms Meade was stabbed.

"There is an expression of almost disbelief on the face of Dawkins."

Jim Sturman QC, defending, told the court Ross had emotional problems which made it difficult for her to deal with female rivalry.

He said: "She was deeply and passionately in love with this young man who was bad for her and bad for Tracy.

"It was an awful and destructive relationship."

The victim's mother, Irene Ford, said in a statement: "This has been unimaginable.

"We have been robbed of a much-loved daughter and her daughters have lost a fantastic mother.

"When I heard the news of Tracy's murder part of me died."

Detective Superintendent Noel McHugh, who led the investigation, added: "It is tragically sad and an awful crime. We have had the right result today."